青藏高原东北缘循化盆地中中新世—早上新世黏土矿物及其古气候意义

青藏高原东北缘古气候可能受控于全球变冷、青藏高原隆升及局地地形变化的影响。为解析气候演化过程及驱动因素,本文以青藏高原东北缘循化盆地西沟剖面作为研究对象,在已有古地磁年龄约束基础上,分析了中中新世—早上新世沉积物中黏土矿物的组成和微观形貌特征。结果表明,西沟剖面沉积物中黏土矿物主要由伊利石、蒙脱石、绿泥石和高岭石组成,其中伊利石含量最高,平均为59. 3％;蒙脱石次之,平均为18. 2％,绿泥石平均含量为12. 3％,高岭石平均含量为10. 2％。根据剖面中黏土矿物含量和比值的变化特征,结合循化盆地西沟剖面的沉积速率、孢粉记录、有机质碳同位素和沉积岩地球化学比值,并与深海氧同位素值（δ18O）变化曲线对比,将循化盆地14. 6~5. 0 Ma气候环境演化划分为3个阶段：14. 6~12. 7 Ma,气候干冷期,与北半球冰盖扩展引发的全球性降温事件有关;12. 7~8. 0 Ma,气候相对温暖湿润期,可能与循化盆地周围山体隆升有关,即积石山在~12. 7 Ma隆升至临界高度,成为西风带输送水汽的地形屏障,使得循化盆地内的降水增强;8. 0~5. 0 Ma,气候再次转向干冷期,该阶段气候的干旱化对应于青藏高原在8 Ma左右的快速隆升,高原进一步的隆升阻碍东亚季风西风带的暖湿气流向内陆的输送,从而引起区域干旱化。     

Geochemical investigation and characterisation of Neogene sediments from Valjevo-Mionica Basin (Serbia)

Sediments from the Valjevo-Mionica Basin of Neogene age were investigated in this study using different geochemical methods. The results of the inorganic part analyses suggest that during genesis of the sediments, the contents of Na₂O, B and As changed the most, which indicates that genesis of the investigated sediments was followed by climate changes and volcanic activity. The quantity of other macro- and microelements did not vary significantly. Investigations of the organic substance showed that it is native organic matter, which has been preserved to a significant extent, formed predominantly of algal precursor organisms, which provided good quality, deposited under reducing conditions and that it is at a low maturity level. As it has been established that genesis of the investigated sediments was mostly affected by an arid climate with more intensive salinity, the aim of this study, was also to identify the processes which were the most affected by the change in salinity, as well as biomarkers which are the most sensitive to this change. The results led to the conclusion that the salinity increase in the depositional environment had a primary effect on the preservation of the organic substance and the formation of the mineral searlesite. In addition, it was noticed that besides the Sq/nC₂₆ ratio, the parameters nC₁₇/nC₂₇, CPI and Ph/nC₁₈, are also significantly sensitive to salinity change in a depositional environment in case of immature organic substance deposited under reducing conditions.     

Trend-surface analysis of distribution of some heavy minerals in shelf sediments off Mangalore, India

The mineralogy of the 0.062–0.125 and 0.125–0.25 mm fractions of 58 selected samples from a set of 78 samples collected at a spacing of 1–2 km in the inner shelf of Mangalore were studied. Trend-surface analysis of distribution of the heavy minerals, hornblende, muscovite, garnet, and sillimanite, were made to determine the various factors controlling the mineral-distribution patterns in the area. Parts of the area showing contrasting environments were analyzed separately to determine the relationship of the regional and local trends and the various factors controlling the trends. The linear, quadratic and cubic trends for hornblende, garnet, and sillimanite were controlled by the source, westerly river flow and southerly currents; for mica, the response to processes has been the main factor. The mineral distribution in this area is mainly the result of dynamic interaction of process and response elements and a process-response model is suggested.Published with permission of the Director General, Geological Survey of India.     

祁连山党河盆地二道泉剖面新近纪沉积物磁性矿物与古水流及其对盆地水系演化的启示

二道泉剖面(39°01' 34″N, 95°52' 49″E) 位于党河盆地腹地, 呈北南向展布, 长790m, 党河盆地位于祁连山腹地, 是研究青藏高原北部构造沉积与地貌演变的理想地区。本研究在野外地层观测、古流水重建及砾石组分分析的基础上, 对党河盆地中西部二道泉剖面新近系白杨河组顶部-玉门砾岩底部河湖相沉积物进行磁化率测量与等温剩磁实验分析。结果显示, 剖面沉积物的磁性矿物主要有磁铁矿和赤铁矿, 其中赤铁矿相对含量较高。通过对比发现剖面中黄色调粗粒沉积物中的赤铁矿含量显著高于红色调细粒沉积物。砾石成分和向南的古流向结果显示, 白杨河组晚期至玉门砾岩早期地层沉积物主要源自于二道泉剖面北部的野马南山地区。磁化率及沉积相揭示出源区物理风化作用逐渐增强, 赤铁矿是从源区直接搬运沉积的, 而非自生形成; 这一过程未受外界气候环境的影响, 赤铁矿多为碎屑, 岩石颜色发黄。综合分析认为, 二道泉剖面疏勒河组到玉门砾岩组早期沉积时党河盆地发育一条规模逐渐变大、水动力逐渐变强的由北向南流动的河流, 现今流向为北西向的党河水系可能最早在新近纪玉门砾岩沉积中期以后才形成。

     

Clay-mineral distribution patterns in late Neogene fluvial sediments of the Subathu sub-basin,central sector of Himalayan foreland basin: implications for provenance and climate

The late Neogene (6–0.5 Ma) fluvial succession of the Subathu sub-basin, a part of the Himalayan foreland basin, comprises a 2.4-km-thick pile of conglomerate, grey and buff sandstone, and mudstone, representing Middle and Upper Siwalik subgroup. This basin is filled mainly by major trunk and piedmont drainage, which are nearly perpendicular to each other. The clay-mineral assemblages of this sedimentary succession have illite (7–82%), smectite (0–90%), chlorite (2–23%) and kaolinite (1–13%). The grey sandstones have moderate to abundant smectite (23–90%), whereas the buff sandstones have abundant illite (66–79%) and low to absent smectite (0–14%). The mudstones that dominates the succession (>50%) have clay-mineral assemblages similar to grey and buff sandstones, or intermediate proportion. The temporal distribution of clay minerals in mudstones shows occasional intense zigzag pattern with either smectite (3–81%) or illite (15–82%) abundance.The smectite-rich grey sandstones and mudstones are deposited by trunk drainage, and the illite-rich buff sandstones and mudstones are deposited by piedmont drainage. The intense zigzag distribution pattern of clay minerals in mudstone indicates interfingering of floods from trunk and piedmont drainages. The interfingering was severe, ranging between 4.8 and 3.36 Ma and between 2.60 and 1.77 Ma, related to tectonic activity. The association of smectite (>36%) bearing mudstones and piedmont source-derived buff sandstone and conglomerate towards the upper part of the section (above 1.77 Ma) suggests either floodwater of trunk drainage over spill on the fringe of piedmont alluvial fan or derivation from smectite bearing Middle Siwalik rocks, exposed due to the activity of an intra-foreland thrust (IFT) in the piedmont zone. The occurrence of smectite and its variable proportion with time suggests its probable derivation not only from the sparsely exposed basic rock in the catchment area but also from siliceous and metamorphic rocks under favourable climatic conditions between 6 and 0.5 Ma.     

Carbon isotope chemostratigraphy and implications of palaeoclimatic changes during the Cisuralian (Early Permian) in the southern Urals,Russia

In order to meet the requirements for potential GSSPs in the Cisuralian Series (Early Permian), isotopic chemostratigraphy from the Carboniferous/Permian boundary to middle Artinskian using bulk carbonates was investigated under high-resolution biostratigraphical and new geochronologic constrains from three GSSP candidate sections at Usolka, Kondurovsky and Dal'ny Tulkas in the southern Urals, Russia. A gradually increasing trend in carbonate carbon isotope (δ¹³C) has been observed in the interval from the base of Asselian to early Sakmarian, which is generally consistent in timing with the increasing development of Glacial III or P1 from the latest Carboniferous to early Sakmarian (Early Permian) which prevailed in southern Gondwana. An excursion with double negative shifts in δ¹³C value is present around the Asselian/Sakmarian boundary in both the Usolka and Kondurovsky sections, which may have great potential to serve as chemostratigraphical marks for intercontinental correlation. The following highly positive excursion of δ¹³C in early Sakmarian indicates the maximium expansion of Glacial III or P1. The negative δ¹³C shift in the middle Sakmarian is possibly related to the quick collapse of Glacial III or P1 on the Gondwanaland. This negative shift is largely correlative with those documented in other areas of Russia, the North American Craton and South China, but further precise biostratigraphical and geochronologic constrains are neccessary to confirm this global signal. The late Sakmarian is characterized by a strong oscillation stage of δ¹³C, which probably indicates a complex climate transition marked by smaller alternating glacial–interglacial transitions during Glacial P2 superimposed on an overall warming trend. The sharp negative δ¹³C shift around the Sakmarian/Artinskian boundary at the Dal'ny Tulkus section is difficult to interpret. This is followed by long-term low values (<?10‰) during the most part of Artinskian Stage. We suggest that the deeply depleted δ¹³C values in the Artinskian at the Dal'ny Tulkas section might result regionally from the enhanced input of organic carbon after the melt-out of ice sheets and the subsequent degradation and isotopic refractionation of the microbial chemosynthetic processes on the buried organic matter.     

Palaeoenvironmental significance of trace fossils from the palaeocene sediments of the Pondicherry area,South India

Sediments of Manaveli Formation (Palaeocene) are studied from the Thondaimanattam quarry section and Manaveli section of the Pondicherry area, which comprises of bioturbated arenaceous limestone-shale and shaly-sandstone sequences respectively. These sediments consist of high density and low diversity trace fossil genera which includes Lanicodichnus, Ophiomorpha, Planolites, Palaeophycus, Skolithos, Thalassinoides, and Polychaetes tubes. The abundance of horizontal structures of the Thondaimanattam quarry section indicate low to moderate wave and current energy in fully marine subtidal environments while the presences of vertical structures of the Manaveli section indicate moderate to high wave and current energy in the marginal marine environment of deposition for the Palaeocene sediments of the Pondicherry area.     

Palynological evidence for the Palaeocene evolution of the forearc basin, Indus Suture Zone, Ladakh, India

The discovery of Permian, Mesozoic and Palaeocene palynomorphs from the Nindam forearc basin, exposed along the Indus Suture Zone in Ladakh, is reported. The palynomorphs are from volcanogenic sandstones and are poorly preserved, distorted and show the effects of abrasion (striation marks). The frequent occurrence of Proxapertites indicates the assemblage is at least Palaeocene in age. The Palaeocene palynomorphs and sediments were transported to the Nindam trough from nearby elevated landward regions (islands). These Palaeocene provenance areas were characterized by an estuarine, nearshore, tropical, warm‐humid environment and were situated at equatorial palaeolatitudes. However, the occurrence of Permian and Mesozoic palynomorphs in the assemblage indicates that the Late Palaeozoic and Mesozoic Tethyan sedimentary rocks exposed along the northern margin of the Indian plate were redeposited into the tectonically active Cretaceous–Palaeocene trench–subduction complex that existed between the Indian and the Asian plates until the collision took place at ～50–60 Ma.     

Drainage morphometry of Himalayan Glacio-fluvial basin,India: hydrologic and neotectonic implications

Morphometric analysis, being widely used to assess the drainage characteristics of the river basins, has been found to be a useful tool to delineate the glacial till covered overburden material as well as to identify areas prone to flash floods in present studies. A number of parameters including the stream frequency, drainage density and drainage texture suggest that the unconsolidated, unstratified and highly permeable glacially deposited overburden till material facilitates the infiltration of snowmelt and rainwater in the Pindari glacio-fluvial basin, Eastern Kumaun Himalaya, India. Likewise, other till overburden covered glacial and proglacial areas of Higher Himalayan regions have been contributing to the groundwater budget. The shape parameters further suggest that the sub-basins with higher form factor are more prone to flash floods. Besides this, the anomalies in the morphometric parameters have been found to be a useful tool to delineate zones of active tectonics in such areas.     

Heavy metal distribution in sediments of Krishna River basin,India

Suspended and bed sediments collected from the entire region of the Krishna River and its major tributaries were analyzed for heavy metals (V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Pb) by the thin-film energy dispersive x-ray fluorescence technique. There is considerable variation in the concentration of elements towards downstream, which may be due to the variation in the subbasin geology and various degrees of human impact. Suspended particles are enriched in heavy metals throughout the basin relative to bed sediments. The heavy metals are enriched in coarse size fractions (10–90 µm) throughout the Krishna River except its tributary Bhima, where finer fractions (2 µm) dominate. Transition elements correlate very well with each other. There is a striking similarity between the bed sediments of Krishna River and the Indian average. When the annual heavy metal flux carried by the Krishna River was estimated, and viewed in relation to the other major riverine transport, the Krishna is seen to be a minor contributor of heavy metals to the Bay of Bengal.     

Spatio-temporal dynamics of heavy metals in sediments of the river estuarine system: Mahanadi basin (India)

Dynamics of heavy metals in the surface sediments of Mahanadi river estuarine system were studied for three different seasons. This study demonstrates that the relative abundance of these metals follows in the order of Fe > Mn > Zn > Pb > Cr > Ni ≥ Co > Cu > Cd. The spatial pattern of heavy metals supported by enrichment ratio data, suggests their anthropogenic sources possibly from various industrial wastes and municipal wastes as well as agricultural runoff. The metal concentrations in estuarine sediments are relatively higher than in the river due to adsorption/accumulation of metals on sediments during saline mixing, while there is a decreasing trend of heavy metal concentrations towards the marine side. The temporal variations for metals, such as Fe, Mn, Zn, Ni and Pb exhibit higher values during monsoon season, which are related to agricultural runoff. Higher elemental concentrations are observed during pre-monsoon season for these above metals (except Ni) at the polluted stations and for metals, such as Cr, Co and Cd at all sites, which demonstrate the intensity of anthropogenic contribution. R-mode factor analysis reveals that “Fe–Mn oxy hydroxide”, “organic matter”, “CaCO₃”, and “textural variables” factors are the major controlling geochemical factors for the enrichment of heavy metals in river estuarine sediment and their seasonal variations, though their intensities were different for different seasons. The relationships among the stations are highlighted by cluster analysis, represented in dendrograms to categorize different contributing sites for the enrichment of heavy metals in the river estuarine system.     

Palynology and depositional facet of lower Permian (Artinskian) sediments from New Majri opencast mine,Wardha basin,India

Coal samples and the associated sediments from New Majri open cast mine have been analysed palynologically with the following objectives: to date the sediments on the basis of the palyno-assemblage recognised; to carry out an inter-basinal, intra-basinal and Gondwana wide correlation; to interpret the palaeoenvironment and depositional facet of the coal field on the basis of dispersed organic matter analysis and sedimentary facies analysis. Based on the qualitative and quantitative analysis of the spore and pollen content one palyno-assemblage-Scheuringipollenites and Faunipollenites has been recognised which is typical of lower Barakar Formation. This has been correlated with known palyno-assemblages from Wardha basin and other basins in India, while with the Gondwana continents the assemblage has been broadly correlated with early Permian Australian, African, South America palynofloras as well as early Permian palynoflora of Antarctica. Consequently, a tentative late Early Permian, Artinskian age is proposed for the sediments from New Majri open cast mine. Palynological studies also revealed that the peat forming vegetation was mainly composed of gymnosperms represented by glossopterids (Scheuringipollenites, Ibisporites, Platysaccus, Cuneatisporites, Primuspollenites and Sahnites), conifers (Faunipollenites, Striatites, Striatopodocarpites, Verticipollenites, Distriatites) and cordaites (Parasaccites, Plicatipollenites, Crucisaccites, Divarisaccus, Densipollenites). Spores were represented mainly by filicopsids (Horriditriletes, Brevitriletes, Callumispora) and sphenopsids (Latosporites). The relative abundance of structured organic matter implies the existence of a fairly dense cover of vegetation in the hinterland. Anaerobic, reducing, water logged peat-forming conditions have been inferred by the presence of biodegraded organic matter and amorphous organic matter. The charcoal fragments recovered from the present study area reflects a possible wildfire in the accumulated swamps or a wildfire in the hinterland after which the sediments were flushed by fluvial systems into the swamps. The coalfield exhibits horizontal bedding pattern which may be due to deposition by suspension settling or horizontal accretion. Further the alternating high and low energy regime is noticed in the sandstone-shale intercalated beds overlying the coarse grained yellow sandstone which forms the roof of the coal seam. Comprehensively the sediments are deposited as overbank / levee deposits.     

Geochemistry of Renuka Lake and wetland sediments,Lesser Himalaya (India): implications for source-area weathering,provenance, and tectonic setting

The geochemical investigation of sediments deposited in the Renuka Lake basin and its adjoining wetland has shown variation in the distribution and concentration of major, trace and REEs. The major elements are depleted in the lake in relation to wetland and that of Post Archaean Australian, Shale (PAAS), except for CaO which is strikingly in excess and has a dilution effect on SiO₂ and other oxides and trace elements. The Wetland sediments, on the other hand, are enriched in Al₂O₃, Fe₂O₃, K₂O and TiO₂ and the latter three show a positive correlation with Al₂O₃ in both wetland and lake sediments suggesting their association with phyllosilicates and similar source rocks. The enrichment of Y, Zr, Ni, Th, U and Nb in wetland compared to lake and their similarity with PAAS in the former, suggests more clay fractions in the wetland. A high Zr/Hf ratio in wetland and lake sediments and a positive correlation of Zr with Y and HREE indicate Zr control on HREEs. However, higher Zr/Yb and Zr/Th ratios in wetland compared to lake indicate mineral sorting during the process of lighter particles (clays) being trapped in wetland soil. This is also reflected from negative correlation of Gd_N/Yb_N with Al₂O₃ and a strong positive correlation with SiO₂ in wetland sediments. The wetland in this context has a control on lake sediment chemistry. The chondrite normalized REE patterns are essentially the same for lake as well as wetland sediments but abundance decreases in the former. The similarity of pattern with that of PAAS and negative Eu anomaly indicates a cratonic source of sediments. In a plot of the individual samples, wetland samples cluster while lake samples are separated indicating fractionation of lake sediments. A strong positive correlation of La_N/Yb_N with Al₂O₃ and a positive correlation of Zr-∑LREE and Zr-La_N/Yb_N suggest that LREEs are controlled by both phyllosilicates and zircon. The chemical index of alteration (CIA) indices in lake sediments and in wetland are higher than PAAS indicating moderate chemical weathering in the source area. The petrography, lack of felsic magmatic rock fragments, and negative correlation between Zr-(Gd/Yb)_C indicate sedimentary source rocks for the detritus. This is in conformity with the Lesser Himalayan sedimentary sequence belonging to neo-Proterozoic–Proterozoic age and constituting lake catchment of Renuka. The tectonic delineation and discriminant function plots of lake and wetland sediments indicate their cratonic and/or quartzose sedimentary orogenic terrain source that has been deposited in a passive margin setting.     

Early Callovian ingression in southwestern Gondwana. Palaeoenvironmental evolution of the carbonate ramp (Calabozo Formation) in southwestern Mendoza,Neuquen basin,Argentina

The carbonatic sequence of the Calabozo Formation (Lower Callovian) developed in southwestern Gondwana, within the northern area of the Neuquén basin, and is widespread in thin isolated outcrops in southwestern Mendoza province, Argentina. This paper describes the facies, microfacies and geochemical-isotopic analysis carried out in five studied localities, which allowed to define the paleoenvironmental conditions of a homoclinal shallow ramp model, highly influenced by sea level fluctuations, where outer, mid and inner ramp subenvironments were identified. The outer ramp subenvironment was only recognized in the south of the depocenter and is characterized by proximal outer ramp facies with shale levels and interbedded mudstone and packstone layers. The mid ramp subenvironment is formed by low energy facies (wackestone) affected by storms (packstones, grainstones and floatstones). The inner ramp subenvironment is the most predominant and is characterized by tidal flat facies (wackestones, packstones and grainstones) over which a complex of shoals (grainstones and packstones) dissected by tidal channels (packstone, grainstones and floatstones) developed. In the north area, protected environment facies were recorded (bioturbated wackestones and packstones). The vertical distribution of facies indicates that the paleoenvironmental evolution of the Calabozo Formation results from a highstand stage in the depocenter, culminating in a supratidal environment, with stromatolitic levels interbedded with anhydrite originated under restricted water circulation conditions due to a progressive isolation of the basin. δ¹³C and δ¹⁸O values of the carbonates of the Calabozo Formation suggest an isotopic signature influenced by local palaeoenvironmental parameters and diagenetic overprints. The δ¹³C and δ¹⁸O oscillations between the carbonates of the different studied sections are related with lateral facies variations within the carbonate ramp accompanied with dissimilar reactivities in relation to diagenetic fluids. The δ¹⁸O values of all sections exhibit a rather broad scatter which may be attributed to diagenesis and recrystallisation while the carbon isotopic composition has been less affected by those processes. Carbon isotope system has best retained the primary isotopic signal and δ¹³C values (0–3.9‰) are within the Callovian isotope range. The ⁸⁷Sr/⁸⁶Sr ratios of the bulk carbonates of El Plomo creek, La Vaina creek and Potimalal River sections are in agreement with the Callovian seawater Sr-isotope curve.     

塔里木盆地西南坳陷中新统安居安组辫状河三角洲沉积

在塔里木盆地西南坳陷中新统安居安组中发现一套辫状河进积到内陆湖泊中而形成的辫状河三角洲沉积体。三角洲3层结构清楚,可明显地分为辫状河三角洲平原亚相、辫状河三角洲前缘亚相及前三角洲亚相3部分,可识别出7种微相。辫状河三角洲特征显著,明显有别于扇三角洲和曲流河三角洲。辫状河三角洲的主体是由含砾砂岩及中、粗粒砂岩组成的辫状河道砂坝及水下分流河道砂坝,单一砂坝呈下粗上细的透镜状,透镜体最大厚度为0．7～3．5m不等。垂向上许多砂坝透镜体相互叠置而成厚的砂体。三角洲中交错层理丰富,其中尤以水道砂坝侧向迁移加积而形成的侧积交错层异常发育为标志。辫状河三角洲砂体是重要的油气储集场所。     

Palaeoenvironmental reconstruction of a glaciotectonized Early and Middle Pleistocene land surface, palaeosol and river sediments: Great Sampford, Suffolk, England

Glaciotectonized sediments and palaeosol at Great Sampford, western Suffolk, England are reconstructed to their original positions in order to determine the form of the original land surface and the associated soil development. The restored stratigraphy consists of Early Pleistocene Kesgrave Sands and Gravels which were deposited by the 'pre-glacial' river Thames, with the Early-Middle Pleistocene Valley Farm Soil developed on a terrace surface. These units are overlain by Sampford Deformation Till and Lowestoft Till, which were formed during the Middle Pleistocene Anglian glaciation. The micromorphological features of the reconstructed soil are interpreted in terms of three climatic cycles, each comprising a period of temperate climate soil formation followed by cold climate soil disruption. The final stage of disruption is associated with the periglacial climate that preceded Anglian glacierization. This pedological reconstruction is the most complex yet recognized from British Early and Middle Pleistocene palaeosols and provides an insight into major climatic oscillations prior to the Anglian Glaciation. The surface upon which the soil developed is one of the oldest terraces of the 'pre-glacial' River Thames that were formed when this river flowed northwards through East Anglia.     

藏南定结地区早中新世淡色花岗岩的形成机制及其构造动力学意义

定结日玛那穹窿位于高喜马拉雅带中段,由花岗片麻岩、变泥质岩、变基性岩及大量淡色花岗岩等组成,经历了角闪岩-麻粒岩相变质作用。为厘定淡色花岗岩的形成机制以及与高级变质岩的关系,我们对淡色花岗岩和高级变质岩进行了全岩元素和Sr和Nd同位素组成和SHRIMP锆石U-Pb地质年代学测试。全岩元素和Sr-Nd同位素测试结果揭示淡色花岗岩具有以下特征:(1)高SiO₂ (>72%),高Al₂O₃ (>12%)和高A/CNK比值 (>1.0);(2)高Rb,低Sr,高Rb/Sr比值(>1.0);(3)高∑REE和明显的负Eu异常;(4)高Sr同位素初始比值(0.7621~0.8846)和低ε_Nd(t)值(-13.0~-20.2)。淡色花岗岩的高Rb/Sr比值和Sr-Nd同位素系统特征表明其形成机制为主要为白云母脱水部分熔融作用,源区为由花岗片麻岩和变泥质岩组成的混合源区。SHRIMP锆石U-Pb年代学研究揭示出定结地区淡色花岗岩具有21.0±0.7Ma和15.8±0.1Ma 2期年龄,花岗片麻岩的锆石变质增生边年龄为22.2±1.4Ma,与该区的榴辉岩退变质年龄一致。这些数据共同表明,花岗片麻岩和 变泥质岩在22~21Ma发生高级变质和深熔作用,形成早期淡色花岗岩岩浆,在~16Ma进一步深熔,形成晚期淡色花岗岩岩浆。     

Paleomires of Eocene lignites of Bhavnagar,Saurashtra basin (Gujarat), western India: Petrographic implications

The Tertiary basins of Gujarat have always been a potential target for their hydrocarbon resources. The lignite resources of the region have also been an important field of research. The present paper presents the results of the petrological study carried out on the lignites of the Saurashtra basin. For this purpose samples were collected from lower and upper lignite seams from the Surkha lignite mine of Bhavnagar, Saurashtra. These samples were subjected to detailed petrographic analysis (both maceral and microlithotype). The study reveals that these lignites are dominantly composed of huminite group macerals while liptinite and inertinite group macerals occur in subordinate amounts. These lignites have attained a thermal maturity up to 0.28-0.30 percent vitrinite reflectance (VRr) which classifies them as ‘low rank C’ coals. Moreover, Bhavnagar lower lignite seam shows relatively less gelification as compared to the upper seam which suffered relatively more biochemical degradation. These lignites are characterized by high gelification index (GI) and low tissue preservation index (TPI).With the help of petrography based facies models an attempt has been made to reconstruct the environment of the paleomire of these lignites.